This invention relates to antenna circuits, particularly spiral antenna circuits which can be comparatively small in size, highly directional and substantially circularly polarized. Such antennas are used frequently, for example, in aircraft for direction finding or warning systems. The basic spiral antenna circuit is well known in the art and can be used in a variety of other systems not necessarily in the small highly directional systems described in U.S. Pat. No. 3,781,898, entitled, "Spiral Antenna with Dielectric Cover", issued Dec. 25, 1973, to this inventor. The antenna circuit of this invention can be applied wherever antenna circuits of a spiral nature are desired, which range in size from thumb size to several feet in diameter.
Archimedian spiral circuits have been useful in many spiral antenna systems. In a two arm spiral circuit having Archimedian spiral, two Archimedian circuit elements originate from adjacent to a center point, 180.degree. out of phase, to a termination some given radius from the center point. The lack of uniformity in frequency response inherent in an Archimedian spiral pattern led to the development of the continuously scaled or logarithmic circuit.
In a two arm logarithmic spiral circuit, two circuit elements originate 180.degree. out of phase from adjacent to a center point and follow a logarithmic path to a termination some given radius from the center point. The logarithmic spiral theoretically has a uniform frequency response at all frequencies. In practice, or select bandwiths, the response is remarkably uniform. However, the effective bandwith is limited by size considerations. Theoretically, a logarithmic spiral should originate at a center point and continue indefinitely for actual frequency independence. Truncating the spiral at some predetermined radius from a given center point limits the frequency independence of the circuit, since the developing lower frequencies are abruptly cut. In practice, abrupt truncation of a spiral causes severe distortions, or truncation effects, in the response characteristics of a spiral antenna due to reflections from the truncated end of the log spiral. Ellipticity in the antenna polarization performance, resulting from undesirable reflections from the truncated spiral, render the antenna unsuitable for high performance requirements of beamwidth uniformity with minimized polarization sensitivity in angular orientations. Other undesirable results from truncating, such as localized thermal energy buildup, are to be avoided.
A logarithmic spiral must, therefore, be effectively terminated and terminated within a reasonable incremental radius. Since the logarithmic spiral is growing exponentially, this latter requirement is of some substance.
One accepted method devised by this inventor and presently in practice, is to transform the expanding logarithmic spiral into a tightly wound Archimedian spiral, thereby allowing a substantial number of turns to be made within a given, small, radial increment allowed for the termination from the coupled end of the logarithmic spiral to the end of the Archimedian spiral. However, the juncture of transformation from the logarithmic spiral to the Archimedian spiral, even though graduated, creates a width and wrap angle discontinuity resulting in an impedence mismatch. This causes distortions, and in particular, an ellipticity at the lower frequencies from a tightly wound terminating spiral. This ellipticity is particularly pronounced as emissions are measured off the central perpendicular axis of a planar radial spiral. In addition, the Archimedian spiral is inherently not uniformly frequency responsive. These and other problems of sensitivity and symmetry led to the conceptualization and development of the antenna pattern of this invention.